This invention relates to a discharge lamp and more particularly to such a lamp comprising a sintered electrode.
Generally, the desired characteristics for a discharge lamp are low ignition voltage, low radio frequency noise, high lumen, and maintaining the high lumen for a sufficiently long period of time. These characteristics depend upon the electron emissive electrode installed in the envelope of the discharge lamp. There are two types of electrodes known in the prior art: one being a coil structure whereby the electron emissive material is contained in the coiled tungsten, and the other being the sintered electrode consisting of a high temperature melting material powder such as tungsten and an emissive material powder such as an alkaline earth metal carbonate.
Although the coil structure produces a discharge lamp having a low ignition voltage, its electron emissive material is easily sputtered by electron or gas ion bombardment. Consequently, its lamp life is reduced significantly. On the other hand, the sintered electrode produces a discharge lamp whereby the electron emissive material is not easily sputtered; this secures long lamp life and low radio frequency noise.
The discharge lamp utilizing the sintered electrode, however, does not maintain a high lumen for a sufficiently long period of time. Due to evaporation of the electron emissive material at high temperatures and some sputtering of the material by electron or ion bombardment, blackening of the lamp's walls occurs. The evaporated or sputtered material attaches to the inner wall of the discharge lamp envelope near the sintered electrode and produces the blackening. As a result, the lumen of the discharge lamp is prevented from passing through the lamp's wall and the lamp loses the desired lumen maintenance.
In order to reduce blackening, an improved sintered electrode has been manufactured by the prior art process of sintering the mixture of an electron emissive powder consisting of barium-calcium tungstate (i.e., sintered emitter) with a base metal powder consisting of tungsten and thorium oxide powder. The barium-calcium tungstate is produced by sintering tungsten oxide, barium carbonate and calcium carbonate in air at high temperatures (i.e., sintered emitter process). Although the prior art sintered electrode formed by this process reduces blackening, the temperature conditions necessary for establishing a high quality of the electrode are critical when the sintered emitter is subsequently sintered with the other powders. Further, the use of thorium oxide has obvious disadvantages due to its radioactive properties.